Config: cache results of get_config (#19605)

`config.get_config` now caches the results and returns the same
configuration if called multiple times with the same arguments
(i.e. the same section and scope).

As a consequence, it is expected that users will always call
update methods provided in the `config` module after changing
the configuration (even if manipulating it as a Python nested
dictionary). The following two examples should cover most
scenarios:

* Most configuration update logic in the core (e.g. relating to
  adding new compiler) should call `Configuration.update_config`
* Tests that need to change the global configuration should use the
  newly-provided `config.replace_config` function.

(if neither of these methods apply, then the essential requirement
is to use a method marked as `_config_mutator`)

Failure to call such a function after modifying the configuration
will lead to unexpected results (e.g. calling `get_config` after
changing the configuration will not reflect the changes since the
first call to get_config).

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